A control circuit of a conventional air blower apparatus that can be used as a ventilation device has the following structure. Firstly, AC power from AC power supply is converted from alternating current to direct current by an AC-DC converter circuit, and smoothed by a capacitor. DC voltage is generated across the capacitor, and input to an inverter via a current detector. Six semiconductors structuring the inverter are mutually operated (switched), to thereby drive a motor. Since the current flowing through the motor flows via the inverter and the current detector, the current is detected by voltage induced across the current detector.
Further, a position sensor is mounted so as to generate a signal corresponding to the position of a rotor based on the rotation of the motor. By the signal from the position sensor, a rotation speed detecting unit detects the rotation speed of the motor. The detected rotation speed is output to an air volume calculation unit and a speed control unit. Further, the air volume calculation unit calculates an air volume generated by a fan connected to the motor based on the current value of the motor detected by the current detecting unit and the rotation speed of the motor detected by the rotation speed detecting unit.
The air volume calculation unit calculates the air volume based on the rotation speed detected by the rotation speed detecting unit and the current detected by the current detecting unit. A target rotation speed calculation unit derives a deviation of the calculated air volume from a target air volume, and calculates a target rotation speed being the target of the motor's operation such that the deviation becomes 0. The speed control unit controls the speed of the motor such that the target rotation speed calculated by the air volume calculation unit is attained. Accordingly, the air volume generated by the fan connected to the motor and the target air volume agree with each other. Thus, the structure for controlling the air volume at a constant value is obtained (for example, see PTL 1).
Further, when an instantaneous power failure occurs, the DC voltage smoothed by the capacitor, i.e., the voltage for driving the motor largely varies. This makes it impossible to control the air volume at a constant value. Accordingly, when the DC voltage drops by an instantaneous power failure, the motor is stopped by immediately stopping the voltage application and discharge of the capacitor is relaxed, to thereby extend the instantaneous power failure supporting time (for example, see PTL 2).